Resistor comprising spaced metal coatings on a resistive layer and traveling wave tube utilizing the same



3,368,1Q3 IVE LAYER Feb. 6, 1968 E. s. THALL RESISTOR COMPRISING SPACEDMETAL COATINGS ON A RESIST AND TRAVELING WAVE TUBE UTILIZING THE SAMEFlled May 20 1964 INVENTOR.

54m: 5., 72AM C014 77/VG 2/) ME 74L cam r/n/os United States PatentRESISTOR (IOMPRISING SPACED METAL COAT- INGS ON A RESISTIVE LAYER ANDTRAVEL- ING WAVE TUBE UTILIZING THE SAME Earle S. Thall, West Orange,N.J., assignor to Radio Corporation of America, a corporation ofDelaware Filed May 20, 1964, Ser. No. 368,965 9' Claims. (Cl. 315-35)ABSTRACT OF THE DISCLOSURE A novel resistive element comprising adiscontinuous conductive layer in contact with a continuous resistivelayer may be made by projectng fluent carbonaceous material, such asgraphite, onto a predetermined portion of a nonconductive base, such asceramic, to form a resistive layer, and then electrolytically depositinga metal on the resistive layer, terminating the depositing before themetal forms a continuous coating. An attenuator for a traveling wavetube may be made by projecting onto a portion of a rotating cylindricalceramic rod to form a resistive annular band thereon, followed by thedeposition of the discontinuous metal layer. The edges of the resistivelayer may be tapered in resistance by slowly moving the carbon sourceaway from the rotating rod during the depositing step.

The present invention relates to an electric resistor.

The resistor of the invention is especially suitable for use in atraveling wave tube, but a resistor of the invention may be usedelsewhere. In a traveling wave tube, an electron beam is projected alongan elongated helix or other slow wave propagating structure forinteraction with waves traveling therealong. In an amplifier, an inputsignal is coupled to one end of the helix and the amplified signal isextracted from the other end of the helix. Due to imperfect impedancematches at the input and output couplings, there is a tendency for wavesto be reflected back and forth along the helix. Such reflection leads toregeneration and oscillations. To avoid this problem, it is customary toprovide a resistive or lossy region or element, preferably .at a pointalong the helix where the amplitude of the forward wave is low, toabsorb or attenuate reflected waves. Such attenuators are usually madeby spraying or otherwise forming a thin coating of some form of carbon,such as graphite, onto a portion of the nonconductive helix support,which may consist of three ceramic rods. However, it is sometimesdiflicult if not impossible to provide suflicient conductance with athin coating on the short length of support rod between adjacent helixturns to effectively attenuate the waves on the helix. The surfaceresistance of the attenuator coating should be less than 100 ohms persquare. The resistivity of carbon is about 800 10 ohm-cm, thus a coatingthickness of about .1 micron is necessary to produce a surfaceresistance of 100 ohms per square. To make the resistance less than 100ohms per square, the thickness must be greater than .1 micron. However,it is diflicult to make a satisfactory adherent carbon coating thickerthan about 0.1 micron, because of the tendency of the carbon particlesto flake off. Also, the thickness of the attenuator coating should bekept to a minimum in order to avoid distortion of the helix. Moreover,it is desirable to taper the resistance at the ends of the attenuator toavoid reflections of waves therefrom. Most of the other availableresistive materials have either too high or too low a resistivity to beuseful as traveling wave attenuators. If one attempts to make a resistorsuitable for this purpose by depositing areas of a metal on anonconductor, the result is either a very low resistance if the areastouch each other or very high resistance if the areas do not touch eachother, and it is very diificult to produce intermediate values ofresistance.

The object of the present invention is to provide an improved electricresistor having any desired intermediate value of resistance.

Another obect is to provide an improved traveling Wave tube attenuator.

Still another object is to provide an attenuator or resistor having acentral region of relatively low resistance and end regions withresistances tapering to very high values.

A further object is to provide a new and improved method of making anattenuator having tapered resistance ends.

In accordance with the invention, a continuous resistive layer is formedon a nonconductive base, a metal is deposited on the resistive layer toform conductive areas thereon, and the deposition is terminated beforethe metal forms a continuous layer.

In the drawing,

FIG. 1 is a side view, partly cut away in section, of a traveling wavetube embodying the present invention;

FIG. 2 is a transverse section view taken on the line 22 of FIG. 1;

FIG. 3 is an enlarged fragmentary detail view taken on the line 3-3 ofFIG. 2; and

FIG. 4 is a plan view showing a portion of the cylindrical attenuator ofFIG. 3 rolled out onto a fiat plane.

FIGS. 1 and 2 show a traveling wave tube, which is conventional exceptfor the improved attenuator of the present invention. The tube comprisesan envelope 1 containing an elongated metal helix 3, supported by threesymmetrically disposed ceramic rods 5, and an electron gun 7, comprisinga thermionic cathode 9, focusing electrode 11, and acceleratingelectrodes 13 and 15, for projecting a beam of electrons through thehelix 3 to a collector (not shown) at the other end of the tube. Each ofthe ceramic rods 5 is coated with an attenuator 17 in contact with someof the turns of the helix 3. The attenuators 17 may be limitedsubstantially to the area of contact with the helix. However, for easeof manufacture, the attenuators are usually provided in the form ofannular bands entirely surrounding the ceramic rods.

As shown in FIGS. 3 and 4, each attenuator 17 comprises a resistivecoating 19 of carbon, or other resistive material, on which amultiplicity or large number of conductive islands of metal have beendeposited in a controlled manner. The function of the conductive islands21is to short out certain portions of the carbon coating and therebyreduce the effective surface resistance of the attenuator as a whole.The effective resistance of the carbon coating 19 and conductive islands21 is also less than the resistance of a thicker carbon coating having athickness equal to the sum of the thicknesses of the coating 19 and theislands 21. The distribution of the islands 21 can be either irregular,as shown in the drawing, or regular. In depositing the metal onto thecarbon coating, care must be taken that most of the metal areas arespaced from each other, and hence constitute true islands. However, someof the areas may touch each other, and form larger islands, withoutunduly reducing the surface resistance of the attenuator. If suflicientareas were to touch each other to form a substantially continuous pathacross the coating, the attenuator would have too low a resistance andthus be inoperative for the purpose intended. As shown in FIG. 3, someof the conductive islands 21 are in contact with the helix 3.

' In making an electric resistor element of uniform lateral resistance,the coating 19 of carbon or other resistive material may be formed byany conventional method, such as spraying, dipping, painting,evaporation, silkscreening, and gas or solid pyrolytic formation. Themetal islands 21 may be deposited on the resistive coating 19 by anatomizing spray, evaporation, sputtering, silkscreening, photo-etchingor electrolytic deposition. The resistive material is preferablycarbonaceous, that is, either containing or composed of carbon. However,other resistive materials may be used, including various semiconductivematerials such as SiC, ZnS, CdSe, InSb, PbTe, Ge (doped) and Si (doped).The resistive coating may also be a very thin evaporated layer of ametal such as tungsten, molybdenum, tantalum and rhenium that does notalloy with the metal chosen for the conductive islands 21. Theconductive islands may be of any conductive metal, such as copper,silver, gold, nickel, platinum, cobalt, tin, cadmium, iron, etc. For useas a traveling wave tube attenuator, the metal must be one that issuitable for use in Vacuum tubes.

In the example shown in FIGS. 3 and 4, the attenuator 17 has a centralportion 17a having an axial length of about 1 inch and substantiallyuniform resistance, and two tapered end portions 17b and 170 each havingan axial length of about /2 inch and a resistance that varies from thatof the central portion to that of the bare ceramic rod 5. The diameterof the ceramic rod may be 100 mils, and the helix 3 may be of milsdiameter wire and have 50 turns per inch.

One method of making the tapered attenuator 17 of FIGS. 3 and 4 is asfollows. First, an elongated ceramic rod 5 is rapidly rotated about itslongitudinal axis and a carbonaceous material is sprayed in a divergentstream from a small source onto the central portion during a selectednumber of rotations of the rod. Then the source is slowly moved awayfrom the rotating rod, thus gradually Widening the sprayed area, untilthe end portions 17b and 17c have been covered, at which time thespraying is stopped. This produces a resistive coating Ha of uniformthickness and resistance in the central portion 17a, and resistivecoatings 19b and 19c on the end areas that taper in thickness from thatof the central portion to Zero at the outer edges, with a correspondingvariation in lateral conductivity.

The carbonaceous material may be applied by spraying the ceramic basewith a solid organic material, such as nitrocellulose, then heating thecoated base to a high temperature, about 900 C. for nitrocellulose, in anonoxidizing atmosphere, to decompose the coating and drive off allnon-carbon constituents thereof. The atmosphere may be either reducing,such as dry hydrogen, or inert, such as nitrogen. A typicalnitrocellulose that has been found suitable has the chemical formula:

This pyrolytic process produces an excellent adherent coating of purecarbon on the ceramic base, which may be of any known ceramic materialhaving a melting point substantially higher than the decompositiontemperature used. As an example, the surface resistance of the carboncoating may be about 400 ohms per square along the central coating 19a,tapering to about a megohm per square at the ends of the end coatings19b and 190.

After the carbon coatings 19a, 19b and 190 have been formed, the coatedceramic rod 5 may be immersed in an electrolytic bath of copper, forexample, and copper islands 21 are plated onto the Carbon coating. Inone example, it was found that a voltage of 3 volts applied to theelectrodes of the bath for 1 /2 minutes produced a combination ofislands 21 and resistive base 19a having the desired surface resistance,about 25 ohms per square. By plating for a longer period of time, thesurface re sistance could be made any lower value, down to that of acontinuous copper layer. Conversely, by plating for a shorter period,the surface resistance could be made any higher value, up to that of thebare carbon coating. Moreover, it was found that although the copperislands deposited on the central coating 19a were approximately uniformin size, the islands deposited on the end coatings 19b and not onlydecreased in size away from the central portion but also terminatedshort of the outer edges of the tapered carbon coatings, as showngenerally in FIG. 4. It is apparent that the conductivity of thethinnest part of the tapered carbon layer was insufiicient toelectroplate any copper thereon. Thus, both the carbon and the coppercoatings in the end portions 171) and 17c of the attenuator are taperedin surface resistance by this method of manufacture. Secondary overlaysof gold over the copper were also found to be satisfactory.

The thicknesses of the carbon and copper coatings are greatlyexaggerated in FIGS. 3 and 4 for clarity of illustration. Actually, thethickness of the central carbon coating 19a is usually less than 0.1micron, and the thickness of the copper islands 21 in the centralportion 17a is even less.

Instead of using the pyrolytic method described above, the carboncoating 19 may be formed on the ceramic rod 5 by spraying the rod with asuspension of graphite particles in a liquid, such as water, and heatingto evaporate the liquid leaving a carbon coating on the rod. The taperedends 1% and 190 may also be formed by spraying the rotating ceramic rodthrough a mask having a rectangular aperture arranged at a skew angle tothe rotating rod, or one parallel to the rod but having ends of taperedWidth. The metal islands 21 may also be deposited on the carbon coating19 by spraying, evaporating or sputtering metal in fluid form through amask having a particular pattern of apertures.

What is claimed is:

1. An electric resistor comprising:

(a) a substantially continuous layer of resistive material; and

(b) a discontinuous conductive layer comprising a large number of spacedmetal coatings on said resistive layer distributed over the surface ofsaid layer, each coating forming a conductive island shorting out aportion of said surface of said resistive layer, whereby the combinedsurface resistance of said two layers is substantially less than thesurface resistance of said resistive layer alone; and

(c) a conductor in contact with at least three of said spaced metalcoatings.

2. An electric resistor comprising:

(a) a base of nonconductive material;

(b) a substantially continuous layer of resistive material on said base;

(c) a discontinuous conductive layer comprising a large number of spacedmetal coatings on said resistive layer distributed over the surface ofsaid layer,

each coating forming a conductive island shorting (c) a discontinuousconductive layer comprising a multiplicity of spaced metal coatings onsaid resistive layer distributed over the surface of said layer, eachcoating forming a conductive island shorting out a portion of saidsurface of said resistive layer, whereby the combined surface resistanceof said two layers is substantially less than the surface resistance ofsaid resistive layer alone.

4. An attenuator for a traveling Wave tube, compris- (a) a ceramic helixsupport rod for said traveling wave tube;

(b) a continuous layer of carbon on a portion of said rod; and

(c) a discontinuous conductive layer comprising a mul tiplicity ofspaced metal coatings on said carbon layer distributed over the surfacethereof; each coating forming a conductive island shorting out a portionof said surface of said carbon layer, whereby the combined surfaceresistance of said two layers is substantially less than the surfaceresistance of said carbon layer alone.

5. A traveling wave tube comprising:

(a) an envelope;

(b) means for projecting a beam of electrons along a predetermined pathin said envelope;

(0) a conductive helix for propagating waves along said path forinteraction with said beam;

((1) helix support means including at least one nonconductive supportrod in said envelope extending along and contacting said helix; and

(e) an attenuator on a portion of said rod intermediate the endsthereof, said attenuator contacting a plurality of turns of said helixand comprising:

(1) a continuous layer of resistive material on said portion of saidrod; and

(2) a discontinuous conductive layer comprising a multiplicity of spacedmetal coatings on said resistive layer each coating forming a conductiveisland shorting out a portion of the surface of said resistive layer,whereby the combined surface resistance of said two layers issubstantially less than the surface resistance of said resistive layeralone.

6. A traveling wave tube comprising:

(a) an envelope;

(b) means for projecting a beam of electrons along a predetermined pathin said envelope;

(c) a conductive helix for propagating waves along said path forinteraction with said beam;

((1) helix support means including at least one ceramic rod in saidenvelope extending along and contacting said helix; and

(e) an attenuator on a portion of said rod intermediate the endsthereof, said attenuator contacting a plurality of turns of said helixand comprising:

(1) a continuous layer of carbon on said portion of said rod; and

(2) a discontinuous conductive layer comprising a multiplicity of spacedmetal coatings on said carbon layer, each coating forming a conductiveisland shorting out a portion of the Surface of said carbon layer,whereby the combined surface resistance of said two layers issubstantially less than the surface resistance of said carbon layeralone.

7. A traveling wave tube as in claim 6, wherein the resistance of saidattenuator is lower at the ends than in the middle thereof, to minimizethe reflection of waves thereby.

8. A traveling wave as in claim 7, wherein the resistance of saidattenuator is about 25 ohms/square for about 1 inch in the middle andtapers at each end to about 1 megohm/square over about the next /2 inch.

9. A traveling wave tube comprising:

(a) an envelope;

(b) means for projecting a beam of electrons along a predetermined pathin said envelope;

(c) conductive means for propagating electromagnetic waves along saidpath for interaction with said beam; and

(d) an attenuator for said Waves comprising:

(1) a continuous layer of resistive material; and

(2) a discontinuous conductive layer comprising a multiplicity of spacedmetal islands on said resistive layer distributed over the surfacethereof, each island shorting out a portion of said surface of saidresistive layer, whereby the combined resistance of said two layers issubstantially less than the surface resistance of said resistive layeralone;

(e) said conductive means being in contact with some of said islands.

References Cited UNITED STATES PATENTS 2,809,321 10/1957 Johnson et al.

2,994,008 7/1961 Geiger et al.

3,200,010 8/1965 Place 338-309 X 3,266,005 8/1966 Balde et a1. 338308HERMAN KARL SAALBACH, Primary Examiner. P. L. GENSLER, AssistantExaminer.

